The invention relates to ultrasound imaging and, more particularly, to correcting for variations in the speed of sound in different tissue types traversed by ultrasound beams during imaging.
Ultrasound imaging systems produce images by detecting and analyzing echoes of high frequency ultrasound beams propagating through and reflected from body tissue. The ultrasound beams are typically transmitted by and then detected by an ultrasound probe, which may comprise a plurality of transducer elements in a transducer array. Each transducer element is driven by electrical signals to cause transmission of an ultrasound beam. To focus the ultrasound beams transmitted by each transducer element onto a focal point proximate a site of interest, transmission time delays (xe2x80x9ctransmission delaysxe2x80x9d) are introduced to the signals driving each transducer element by a beam former so that the transmitted ultrasound beams constructively interfere at the focal point. The location of the focal point is typically selected by an operator of the ultrasound system. Sound waves reflected from the tissue at the site of interest and from tissue and other reflecting bodies between the focal point and the ultrasound probe (referred to as the pass zone) are also detected by the transducer elements in the array. Reflected sound waves are referred to as echo signals. The phases of the detected echo signals are also aligned by the introduction of reception time delays (xe2x80x9creception delaysxe2x80x9d) to the detected signals prior to summing the signals. Images are reconstructed from the summed signals. Static and dynamic ultrasound images may be generated for analyzing the function of an organ and tissue morphology.
Image quality depends on many system-related factors, such as probe acoustical design, front end hardware and imaging frequency, as well as patient-related factors, such as tissue inhomogeneity and target reluctance, for example. Ultrasound imaging is difficult or impossible to perform on about 30% of patients due to patient-related factors.
Tissue inhomogeneity affects image quality because of the differing speed of sound through different types of tissue. The transmission delays for each signal driving each transducer element, as well as the reception delays for bringing echo signals into phase, are computed based on the distance from each transducer element to the focal point or reflecting body in the pass zone and an average speed of sound in body tissue. Sound travels through body tissue at an average speed of sound of 1540 m/s. Zagzebski, James A., Essentials of Ultrasound Physics, Mosby-Year Book, Inc., Missouri, (1996), p. 6. The average speed of sound in body tissue may vary for different individuals and other uniform speeds may be used as the average speed of sound. The range of values for the average speed of sound in body tissue is 1540 m/s plus or minus 3%. Furthermore, sound travels through different types of body tissue at different speeds. For example, the speed of sound through fat is 1460 m/s. The speed of sound through muscle is 1600 m/s. The speed of sound through bone tissue is much faster (3000 m/s for skull bone tissue, for example). The speed of sound will also vary in different organs. For example, the speed of sound in liver tissue is 1555 m/s. In kidney tissue, the speed of sound is 1565 m/s. (We note that there is some variability in the reported speeds of sound in different body tissues and the average speed of sound in body tissue due to variability among individuals).
During transmission of the ultrasound beams, these speed differences may shift the phase of the transmitted ultrasound beams, decreasing the constructive interference at the focal point. Image contrast resolution is thereby decreased. Such phase shifts also increase side lobes of the beam, further decreasing the image quality.
During detection and processing of echo signals, the arrival times of the echo signals are also shifted, decreasing contrast resolution and introducing geometrical errors into the image. For example, with an imaging depth of 10 cm, traversal of 3 cm of fat at a speed of 1460 m/s (instead of the average speed of 1540 m/s), will delay the arrival time of echo signals from the site of interest by about 1.3 microseconds. This can cause an error in location of a site of interest of about 2.5 mm in an image. Such an error could be detrimental in ultrasound guided medical procedures, such as needle biopsies, for example.
Tissue inhomogeneity also causes refraction of the ultrasound beams at the boundaries of tissue regions having different speeds of sound. Refraction may also decrease constructive interference at the focal point. While generally of lesser concern than phase shift in soft tissue, refraction caused by boundaries between bone and soft tissue can also seriously degrade contrast resolution of an ultrasound image.
Efforts to correct for patient-related factors have included analyzing the reflected signal and optimizing the detected echo signals and the reconstructed image based on predefined control parameters, in a manner similar to auto-focusing. In U.S. Pat. No. 4,817,614, for example cross-correlation of ultrasound echo signals received at adjacent transducer elements of an ultrasound probe are used to identify and compensate for tissue inhomogeneities. First, a sectional plane of a subject is scanned. Then, unwanted effects caused by the inhomogeneities are measured for each transducer element, based on cross-correlation of the echo signals received by adjacent transducer elements in the probe. Correction values for the delay times are derived from the measured values and then the delay times are varied based on the correction values.
To correct for tissue inhomogeneity in accordance with the present invention, a speed of sound other than an average speed of sound in at least one selected tissue region in the pass zone is used in conjunction with the boundaries of the selected tissue region to determine focusing delay times (either transmission delays, reception delays or both). In particular, the actual speed of sound in the selected tissue region or a speed between the actual speed of sound and the average speed of sound, may be used. The closer the speed is to the actual speed of sound, the more accurate the correction. Sufficient correction will generally be provided by considering a speed of sound other than the average speed of sound for fat and bone tissue regions, if present. If further correction is desired or necessary, a speed of sound other than the average speed of sound in other tissue regions may be considered, as well.
In accordance with one embodiment of the invention, a method of imaging a site of interest in a body using an ultrasound probe, where the ultrasound probe comprises a plurality of ultrasound transducer elements, is disclosed. The method comprises obtaining an ultrasound image of a pass zone of the body between the site of interest and the ultrasound probe. The ultrasound image includes the site of interest and a plurality of tissue regions in the pass zone. Boundaries of a selected tissue region in the pass zone are determined from the image. Respective focusing delay times for each transducer element associated with an ultrasound beam passing through the selected tissue region are computed. The focusing delay times are computed based, at least in part, on a speed of sound in the selected tissue region other than an average speed of sound in body tissue, and the determined boundaries of the selected tissue region. An ultrasound imaging scan of the pass zone is then conducted employing the computed focusing delay times.
The focusing delay times may be computed by determining a respective propagation time between each transducer element and respective points in the pass zone based, at least in part, on the speed of sound in the selected tissue region other than the average speed of sound, and a respective distance traveled by an ultrasound beam through the selected tissue region, based on the determined boundaries. Refraction of the ultrasound beams may be considered in computing the focusing delay times.
The speed of sound in the selected tissue region other than an average speed of sound in body tissue is preferably xe2x80x9caboutxe2x80x9d the speed of sound in the tissue type of the selected tissue region. In the present application, what is considered xe2x80x9cabout the speed of soundxe2x80x9d varies for different tissue types. For example xe2x80x9cabout the speed of soundxe2x80x9d in fat tissue is a speed within a range of plus or minus 3% of 1460 m/s. More preferably, the speed is in a range of plus or minus 1% of 1460 m/s. Most preferably, the speed of sound in fat tissue of 1460 m/s is used. For muscle tissue, xe2x80x9cabout the speed of soundxe2x80x9d is a speed in a range of plus or minus 2% of the speed of sound of 1600 m/s. More preferably, the speed is in a range of plus or minus 1% of 1600 m/s and even more preferably, the speed of sound in muscle tissue of 1600 m/s is used. For liver tissue and kidney tissue, xe2x80x9cabout the speed of soundxe2x80x9d are speeds in ranges of plus or minus 2.0% of the speeds of sound of 1555 m/s and 1565 m/s, respectively. More preferably, the speeds are in a range of plus or minus 1.0%, and more preferably plus or minus 0.5% of 1555 m/s and 1565 m/s, respectively. Most preferably, the speeds of sound of 1555 m/s and 1565 m/s are used for liver tissue and kidney tissue, respectively. For bone tissue, xe2x80x9cabout the speed of soundxe2x80x9d is a speed in a range of plus or minus 40% of 3000 m/s. More preferably, the speed is within a range of plus or minus 20% of 3000 m/s. A speed in a range of plus or minus 10% of 3000 m/s is even more preferred, and most preferably, 3000 m/s is used. The speed of sound in brain tissue has been observed to be about 1570 m/s. For brain tissue, xe2x80x9cabout the speed of soundxe2x80x9d is a speed in a range of plus or minus 3% of 1570 m/s. A speed in a range of plus or minus 1.0% of 1570 m/s may also be used. 1570 m/s may be used as well.
The selected tissue region may be a fat, bone, muscle and/or organ tissue region. As mentioned above, sufficient correction may generally be obtained by selecting fat and bone tissue regions, if present. Further correction may be obtained by using the speed of sound in muscle tissue, instead of the average speed of sound in body tissue for non-selected soft tissue regions. Even further correction may be provided by determining the boundaries of muscle and organ tissue regions, if present, and considering the speeds of sound in those tissue regions, as well.
The focusing delay times may be transmission delays, which are computed for each transducer element transmitting an ultrasound beam through the selected tissue region. Each transmission delay is computed such that ultrasound beams transmitted by respective transducer elements will constructively interfere at a selected focal point.
The computed focusing delay times may also be reception delays, which are computed for each transducer element receiving an ultrasound beam passing through the selected tissue region. The reception delays are computed such that ultrasound beams reflected from reflecting bodies in the pass zone are in phase after detection.
The focusing delay times may be computed, at least in part, by conducting a ray calculation between a point in the pass zone and each transducer element transmitting an ultrasound beam through, and/or receiving an ultrasound beam passing through, the selected tissue region. A distance traveled in the selected tissue region for each ultrasound beam corresponding to a ray may then be determined.
The ultrasound image may be obtained from an ultrasound imaging scan employing initial focusing delay times based on an average speed of sound in body tissue. The respective computed transmission and or reception delays may also be computed by determining a respective adjustment to the initial focusing delay times for each transducer element transmitting or receiving an ultrasound beam passing through the selected tissue region. The adjustments may be based, at least in part, on a phase shift for each ultrasound beam, caused by passage through the selected tissue region.
The boundaries of the selected tissue region may be determined by segmentation. A three dimensional boundary of the selected tissue region may be determined based on a plurality of ultrasound images of the pass zone, where each image comprises a different sectional plane through the site of interest. Three dimensional boundaries of the selected tissue region may also be determined by three dimensional ultrasound imaging of the pass zone.
If the initial correction process does not provide sufficient improvements in image contrast resolution, the process may be repeated using the corrected image resulting from an ultrasound imaging scan employing the computed focusing delay times. The corrected image may be used to determine the boundaries of the selected tissue region, which are then used to compute new focusing delay times for a subsequent corrected image. The process may be repeated any number of times with each subsequent corrected image. Each corrected image should be an improvement over the prior image.
In accordance with other embodiments of the invention, an ultrasound imaging system, software stored on a machine readable medium for controlling an ultrasound imaging system and a method for determining focusing delays, are also disclosed.
In accordance with yet another embodiment of the invention, an ultrasound probe comprising high and low frequency transducers is also disclosed.